Fuel trim system for a multiple chamber gas turbine combustion system

ABSTRACT

A fuel trimming unit has been adapted to each combustion chamber in a multi-chamber combustor of an industrial gas turbine. The fuel trim unit matches the combustion air flow into each combustion chamber. This adjusts the fuel-air mixture to account for air flow variations into the individual combustion reaction zones of each combustion chamber. Because of the fuel trim unit, a uniform fuel-air mixture is provided to each combustion chamber in a gas turbine. The fuel trim unit is adjusted using sensor inputs of the fuel flow rate to each combustion chamber, the dynamic pressure in each combustion chamber, and the turbine exhaust temperature.

This is a divisional of application Ser. No. 08/168,570 filed on Dec.16, 1993, now U.S. Pat. No. 5,423,175, which is a division of priorapplication Ser. No. 07/996,934, filed on Dec. 30, 1992 now issued U.S.Pat. No. 5,319,931.

FIELD OF THE INVENTION

This invention relates to fuel control systems for gas turbines. Inparticular, this invention relates to fuel trimming systems forindustrial gas turbines having plurality of combustion chambers.

DESCRIPTION OF THE RELATED ART

Industrial gas turbines are required to perform at higher and higherefficiencies while producing less and less undesirable air pollutingemissions. Higher efficiencies in gas turbines are generally achieved byincreasing overall gas temperature in the combustion chambers of the thegas turbine. Emissions are reduced by lowering the maximum gastemperature in the combustion chamber. The demand for higherefficiencies which results in hotter combustion chambers conflicts to anextent with the regulatory requirements for low emission gas turbines.

The primary air polluting emissions produced by gas turbines burningconventional hydrocarbon fuels are oxides of nitrogen (NOx), carbonmonoxide (CO) and unburned hydrocarbons (UHC). The oxidation ofmolecular nitrogen in gas turbines increases dramatically with themaximum hot gas temperature in the combustion reaction zone of eachcombustion chamber. The rate of chemical reactions forming oxides ofnitrogen is an exponential function of temperature. The volume of NOxemissions can be very great even if the hot maximum temperature isreached only briefly. A common method for reducing NOx emissions is tolower the maximum hot gas temperature in the combustion chamber bymaintaining a lean fuel-air ratio.

If the fuel-air mixture in a combustion chamber is too lean, thenexcessive emissions of carbon monoxide and unburned hydrocarbon occur.CO and UHC emissions result from incomplete fuel combustion. Generationof these emissions usually occurs where the fuel-air mixture excessivelyquenches combustion in the reaction zone. The temperature in thereaction zone must be adequate to support complete combustion or thechemical combustion reactions will be quenched before achievingequilibrium. Unfortunately, prematurely quenched combustion too oftenoccurs in current low-NOx combustors that operate with fuel-air mixturesnear the lean limit of flammability.

The rates of CO and UHC emission generation due to combustion quenchingare non-linear functions of reaction zone temperature and peak sharplyat the lean fuel-air ratio limit of flammability. To minimize CO and UHCemissions, the reaction zones of gas turbine combustors should haveadequate fuel-air mixtures to avoid the lean limit of flammability.However, combustors must still operate with lean fuel-air mixtures toreduce NOx emissions. To balance the conflicting needs for reduced CO,UHC and NOx emissions, extremely precise control is required over thefuel-air mixture in the reaction zones of the combustors in anindustrial gas turbine.

The fuel-air ratio in each combustion chamber of a gas turbine should bethe same. A constant fuel-air mixture in each combustor allows themixture to be maintained at the lean ratio that best reduces CO, UHC andNOx emissions. In addition, uniform fuel-air ratios among chambersensures a uniform distribution of temperature among the combustors of agas turbine. A uniform distribution of temperature and pressure reducesthe thermal and mechanical stresses on the combustion, turbine and otherhot stream components of the gas turbine. A reduction in these stressesprolongs the operational lives of combustor and turbine parts. Peak hotgas temperature in some combustion chambers (but not others) increasesthermal stresses and reduces the strength of materials in the hotterhigh fuel-air ratio chambers and turbine parts immediately downstream ofthose chambers.

It has proven extraordinarily difficult to achieve truly uniformtemperature and pressure distribution in multiple combustion chambers ofindustrial gas turbines. For example, the air flow distribution incombustion chambers is perturbed by variations in the components of thecombustion chambers and their assembly. These variations are due tonecessary tolerances in manufacturing, installation and assembly of thecombustor and gas turbine parts. In addition, the air flow paths areirregular approaching the combustion system from the compressor andexiting at the combustor discharge to the turbine. These irregular pathsaffect the air flow through the combustor and cause a non-uniform airflow distribution in the combustors. For example, localized air flowresistance is caused by the lines for turbine bearing lube oil in thecompressor discharge air flow path. The irregular air flow distributionamong combustion chambers affects the fuel-air ratio differently in eachcombustion chamber. Variations in the air flow in each combustionchamber make it difficult to maintain constant fuel-air ratios in allcombustion chambers.

Prior fuel systems for multiple combustion chamber industrial gasturbines provide uniform fuel flow distribution among the chambers.These systems have a common control that meters the same rate of fuel toeach chamber. These systems do not trim the fuel flow to each combustionchamber to maintain a uniform fuel-air ratio in each chamber.Accordingly, these prior fuel systems cannot maintain a truly uniformfuel-air ratio in all combustion chambers when the air flow is notuniformly distributed among combustion chambers.

SUMMARY OF THE INVENTION

The present invention is a control system for trimming the fuel flow toeach combustion chamber in a multiple chamber gas turbine combustionsystem. The fuel flow distribution among chambers is trimmed to matchthe air flow distribution to obtain a uniform distribution of fuel-airratios among chambers. Optimal fuel trimming equalizes the fuel-airratio in all chambers regardless of uncontrolled chamber-to-chambervariations in the air flow.

The control signals to the fuel trimming system are: (1) individualcombustion chamber fuel flow rates, (2) individual combustion chamberdynamic pressure levels, and (3) gas turbine exhaust temperaturedistribution around the entire turbine discharge. These signals may beused individually or in combination to determine the optimum fuel flowdistribution set by the trimming system. Conventional instrumentationfor each combustion chamber is used to obtain these control signals forthe fuel trimming system. These instruments are well known in the gasturbine industry and have proven reliable.

One embodiment of the invention is a gas turbine comprising acompressor, a multi-chamber combustor receiving pressurized air from thecompressor, a turbine drivingly connected to the compressor andreceiving exhaust from the combustor, a fuel system for providing fuelto each chamber of the multi-chamber combustor, where the fuel systemtrims the fuel to individual chambers to match the air flow to eachchamber.

Similarly, the invention in another embodiment is a combustion sectionof a gas turbine having a plurality of chambers, at least one of thechambers comprising: at least one combustion reaction zone receiving airfrom a compressor and fuel from a fuel distributor; the fuel distributorhaving a fuel trim orifice and a fuel trim valve, the fuel trim valvefor the at least one chamber being individually set to trim the flow offuel to the chamber.

The advantages provided by the present invention include uniformdistribution of fuel-air ratios among multiple combustion chambers tominimize the emissions of objectionable air pollutants in the gasturbine exhaust, including nitrogen oxide, carbon monoxide, and unburnedhydrocarbons over the entire load range of a gas turbine. In addition,uniform distribution of fuel-air ratio prolongs the operational life ofthe hot stream components of the gas turbine.

An object of this invention is to provide a method for obtaining auniform distribution of fuel-air ratio among all the combustion chambersof a multiple chamber combustion system in an industrial gas turbine. Inparticular, it is an objective to maintain a uniform fuel-air ratio ineach chamber of a multiple chamber gas turbine combustion chambersystem, when air flow is not uniformly distributed among the combustionchambers. It is a further objective of this invention to trim the fuelflow distribution among the combustion chambers to match variations inair flow to each chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings contain reference numerals used in thefollowing detailed description of an embodiment of the presentinvention.

FIG. 1 is an elevation view of a gas turbine engine shown in partialcross section;

FIG. 2 is a block diagram of a fuel trimming system in accordance withthe present invention;

FIG. 3 is a schematic diagram of the instrumentation and control systemfor the fuel trim system shown in FIG. 2; and

FIG. 4 is a schematic diagram of a computerized instrument.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a gas turbine 12 that includes a compressor 14, acompressor exhaust duct 15, multiple combustion chambers (one shown) 16and a turbine 18 represented by a single blade. Although it is notspecifically shown, it is well known that the turbine is drivinglyconnected to the compressor along a common axis. The compressorpressurizes inlet air is turned 19 to the combustor where it cools thecombustor and provides air for combustion.

The plurality of combustion chambers 16 are located about the peripheryof the gas turbine. In one particular gas turbine, there are fourteenchambers disposed about the periphery of the gas turbine. A transitionduct 20 connects the outlet of a particular combustion chamber to theinlet of the turbine to deliver the hot products of the combustionprocess to the turbine.

The invention is particularly useful in a dual stage, dual mode low NOxcombustor of the type described in U.S. Pat. No. 4,292,801. As describedin that patent and shown in FIGS. 1 and 2, each combustion chamber 16comprises a primary or upstream combustion reaction zone 24 and asecondary or downstream combustion reaction zone 26 separated by aventuri throat region 28. Each combustion chamber is surrounded by acombustor flow sleeve 30 that channels compressor discharge air flow tothe chamber. The chamber is further surrounded by all outer casing 31bolted to the turbine casing 32.

Primary fuel nozzles 36 deliver fuel to the upstream reaction zone 24and are arranged in an annular array around a central secondary fuelnozzle 38. In one model gas turbine, each combustion chamber may includesix primary nozzles and one secondary nozzle. Fuel is delivered to thenozzles from a centralized annular fuel manifold 42. From this manifold,fuel is piped 43 through a filter and to fuel distributors for theprimary 24 and secondary 26 combustion reaction zones. The secondarydistributor 44 routes fuel to the secondary fuel nozzle 38 and theprimary distributor 45 is an annular piping unit that routes fuel to theprimary nozzles 36.

Each distributor has an associated fuel trim unit. The secondary fueltrim unit 46 for the secondary distributor has an adjustable valve 60operated by a technician. The technician reads sensor signals frompressure, temperature, and fuel flow rate sensors. A similar primaryfuel trim unit 48 trims the fuel flow to the primary nozzles. Ignitionin the primary combustion chamber is caused by a spark plug 48 and byadjacent combustion chambers through crossfire tubes 50.

FIG. 2 shows a fuel trim system as applied to a dual stage, dual modelow NOx combustion system as described in U.S. Pat. No. 4,292,801. Themultiple combustion chambers 16 of a gas turbine combustion system arelabeled chambers 1, 2, 3 to N, where N is the total number of combustionchambers in the combustion system. Combustion reactions occur in boththe primary and secondary reaction zones in each chamber, eitherindependently or in combination. Fuel and air are introduced into thereaction zones of the combustion chamber, combustion occurs, and fuel isoxidized releasing heat which results in a temperature and pressure risein the combustion gases. In a typical application, the fuel is ahydrocarbon, such as methane, CH₄, end oxidation products of combustionto equilibrium are primarily carbon dioxide, CO₂, and water, H₂ O. Thecombustion products are usually diluted with excess air provided asdilution air through the combustor from the compressor.

The distribution of hot gas temperature within the reaction zones 24, 26of all combustion chambers 16 depends upon the fuel-air ratios in thereaction zones of each chamber. The distribution of hot gas temperatureand pressure in the flow of combustion gases exiting the combustionchambers and entering the first stage of the turbine 18 depends upon theoverall fuel-air ratio in each of the combustion chambers.

In general, the air flow rate will vary to each of the combustionchambers. The fuel flow to each reaction zone in each chamber is trimmedto account for the air flow variation. The fuel flow to each individualcombustion chamber primary and secondary reaction zone is trimmed, i.e.,raised or lowered, relative to the average fuel flow to all chambers.This trimming is accomplished by the fuel trim system 46, 48 that matchthe fuel flow to the air flow for each combustion chamber.

The technician adjusts the fuel flow rate to both the secondary andprimary fuel nozzles in response to conditions monitored. Thisadjustment can be made at any time during operation of the gas turbine,but will usually be done at installation or during overhaul of the gasturbine. By trimming the fuel rate individually to each combustionchamber, the fuel flow rate can be matched to the individual air flowrate in each combustion chamber to maintain a constant fuel-air ratio ineach chamber.

FIG. 3 shows a configuration for the fuel trim unit 46 or 48 for asingle combustion chamber. A fuel trim control valve 60 adjusts the fuelflow rate to each reaction zone 24, 26 by varying the flow resistance ofthe fuel supply line to each zone of each combustion chamber.

The measured parameters used to set the fuel trim control valve are (1)measured fuel flow rate to each fuel trim system; (2) measuredcombustion chamber dynamic pressure, and (3) measured gas temperaturedistribution in the gas turbine exhaust. A technician monitorsindividual combustion chamber dynamic pressures, the distribution of gasturbine exhaust temperature and individual combustion chamber fuel flowrates.

A conventional fuel flow meter 64 is included in the fuel trim system tomeasure the fuel flow rate to each reaction zone in each combustionchamber. This measured fuel flow rate is used to maintain a desired fuelflow split between the primary and secondary reaction zones in eachcombustion chamber. The total fuel flow to each chamber and the fuelflow to each reaction zone is adjusted via valves 60 to match air flowdistribution to the chamber.

A conventional dynamic pressure sensor 66 (FIG. 1) in each combustionchamber provides chamber pressure measurements that are displayed to thetechnician adjusting the fuel trim valves 60. Similarly, a conventionalexhaust thermocouple array 68 (FIG. 1 shows just one thermocouple probe)provides the technician with data regarding the temperature distributionof exhaust gases exiting the turbine. Given the data from the pressureand temperature sensors and the fuel flow meters, a technician canadjust the fuel trim valves to each of the reaction zones to eachcombustion chamber. In this manner, the fuel flow to each reaction zonecan be trimmed to maintain a uniform fuel-air mixture in all combustionchambers.

FIG. 4 shows a detailed diagram of a fuel trim unit in an alternativeembodiment where the fuel trim valves are under computer, rather thanmanual, control. A computer 70 monitors the sensor data to continuouslytrim fuel flow to maintain a uniform fuel-air ratio in each combustionchamber. In the manual method, the trim valves 60 are set to a fixedposition at installation and may be adjusted during maintenance. Thismanual operating method suffices because the air flow distribution isnot expected to change significantly over the life of the turbine. Thus,once the fuel flow is trimmed to match each chamber at installation ofthe gas turbine, it is reasonable to expect that the fuel-air match willbe valid for the life of the gas turbine. However, continuous computercontrolled fuel trimming could be desirable where truly exact fuel-airtrimming is desired.

The computer controller 70 is a conventional controller such as the MARKV controller computer for industrial gas turbines sold by the assigneeGeneral Electric Company. The computer receives sensor data from theexhaust thermocouple array 68 and chamber dynamic pressure sensors 66.Similarly, the computer controller receives fuel flow rate data from theflow meters 64 for each reaction zone. Using the sensor inputs, thecomputer controller activates a solenoid 72 that adjusts the fuel trimvalve 65.

The fuel trim valve 60 in both manual and computer controlledembodiments includes the trim valve 65 and a trim fuel orifice 74 inseries and a parallel main fuel orifice 76. The parallel main and trimfuel orifices protect the gas turbine from unintentional extremevariations in fuel flow distribution among combustion chambers. If allfuel flowed through the fuel trim orifice, then extreme fuel variationsmight occur while the fuel flow is being trimmed to match air flow. Theparallel main and trim orifices limit the maximum fluctuation of fuelflow due to the fuel trim unit.

The invention has been described as applied to a two stage low NOxcombustion system. However, it could be applied to a single stage lowNOx combustion system, a single stage conventional combustion system orany other gas turbine combustion system provided the system usesmultiple combustion chambers.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A combustion section of a gas turbine having aplurality of chambers, at least one of said chambers comprising:at leastone combustion reaction zone receiving air from a compressor and fuelfrom a fuel distributor; said fuel distributor having a fuel trimorifice and a fuel trim valve, said fuel trim valve for said at leastone of said chambers being individually adjustable to trim the flow offuel to the chamber, and means to operate said fuel trim valve toregulate to the air flow to said chamber to maintain a selected fuelair-ratio in said chamber.
 2. A combustion section as in claim 7 furthercomprising:a pressure sensor detecting a dynamic gas pressure in thechamber; a temperature sensor detecting an exhaust temperature of thecombustion section, and a computer controller means for operativelyadjusting the fuel trim valve in response to the dynamic gas pressuredetected by the pressure sensor and the exhaust temperature detected bythe temperature sensor.
 3. A combustion section of a gas turbine havinga plurality of chambers, at least one of said chambers comprising:atleast one combustion reaction zone receiving air from a compressor andfuel from a fuel distributor; said fuel distributor having a fuel trimorifice and a fuel trim valve, said fuel trim valve for said at leastone of said chambers being individually adjustable to trim the flow offuel to the chamber; a pressure sensor detecting a dynamic gas pressurein the chamber; a temperature sensor detecting an exhaust temperature ofthe combustion section, and a controller means for operatively adjustingthe fuel trim valve in response to the dynamic gas pressure detected bythe pressure sensor and the exhaust temperature detected by thetemperature sensor.
 4. A gas turbine as in claim 1 wherein said fueltrim valve continuously adjusts the trim flow to individual chambers tomatch the air flow to each chamber.
 5. A gas turbine as in claim 1wherein each said combustor includes primary and secondary reactionzones and each member zone has an associated fuel control trim valve.